Magnetic recording medium having controlled coercive force ratio

ABSTRACT

Provided is a magnetic recording medium including: a recording layer containing a powder of particles containing ε iron oxide, in which a ratio ((Hc(50)/Hc(25))×100) of a coercive force Hc(50) measured in a thickness direction of the magnetic recording medium at 50° C. and a coercive force Hc(25) measured in the thickness direction of the magnetic recording medium at 25° C. is 95% or greater, the coercive force Hc(25) is 200 kA/m or greater, and a squareness ratio measured in a transport direction of the magnetic recording medium is 30% or less.

TECHNICAL FIELD

The present technology relates to a magnetic recording medium.

BACKGROUND ART

In a magnetic tape for high-density recording supporting LinearTape-Open 6 (LTO-6), hexagonal barium ferrite magnetic powder is usedinstead of needle-like magnetic powder. Besides barium ferrite magneticpowder, cubic CoMn spinel ferrite magnetic powder (see Patent Literature1 for example), ε-Fe₂O₃ magnetic powder (see Patent Literature 2 forexample), and the like have been reported as magnetic powders forachieving high-density recording.

CITATION LIST Patent Document

-   Patent Document 1: Japanese Patent No. 4687136-   Patent Document 2: Japanese Patent No. 5013505

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Meanwhile, in recent years, library devices using magnetic tape forarchival purposes are beginning to show promise. In a library device,because magnetic tape is kept at a constant temperature of approximately50° C., a lowering of the signal-noise ratio (SNR) in a high-temperatureenvironment due to magnetization loss by heat is a concern. Innext-generation magnetic tape having a higher recording density, becausethe magnetic particles contained in the recording layer are atomizedfurther, the lowering of the SNR in a high-temperature environment isthought to become more significant.

An object of the present technology is to provide a magnetic recordingmedium capable of suppressing SNR degradation in a high-temperatureenvironment.

Solutions to Problems

The present technology to achieve the above object relates to a magneticrecording medium including: a recording layer containing a powder ofparticles containing ε iron oxide, in which a ratio((Hc(50)/Hc(25))×100) of a coercive force Hc(50) measured in a thicknessdirection of the magnetic recording medium at 50° C. and a coerciveforce Hc(25) measured in the thickness direction of the magneticrecording medium at 25° C. is 95% or greater, the coercive force Hc(25)is 200 kA/m or greater, and a squareness ratio measured in a transportdirection of the magnetic recording medium is 30% or less.

Effects of the Invention

According to the present technology, SNR degradation in ahigh-temperature environment can be suppressed. However, theadvantageous effects are not necessarily limited to the one describedhere, and may also be any of the advantageous effects described in thisdisclosure or dissimilar advantageous effects.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-section diagram illustrating a configuration of amagnetic recording medium according to an embodiment of the presenttechnology.

FIG. 2 is a cross-section diagram illustrating a configuration of amagnetic particle.

FIG. 3 is a cross-section diagram illustrating a configuration of amagnetic particle.

FIG. 4 is a cross-section diagram illustrating a configuration of amagnetic recording medium.

FIG. 5 is a cross-section diagram illustrating a configuration of amagnetic recording medium.

FIG. 6 is a cross-section diagram illustrating a configuration of amagnetic recording medium.

MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present technology will be described in thefollowing order.

Configuration of magnetic recording medium

Method of manufacturing magnetic recording medium

Effects

Modifications

[Configuration of Magnetic Recording Medium]

A magnetic recording medium according to one embodiment of the presenttechnology is, as illustrated in FIG. 1, provided with an elongatedsubstrate 11, a foundation layer (non-magnetic layer) 12 provided on oneof the principal planes of the substrate 11, and a recording layer(magnetic layer) 13 provided on the foundation layer 12. As necessary,the magnetic recording medium additionally may be provided with aprotective layer (not illustrated), a lubricant layer (not illustrated),and the like provided on the recording layer 13. Also, as necessary, abackcoat layer 14 provided on the other principal plane of the substrate11 additionally may be provided. The magnetic recording medium has anelongated shape, and is transported in the lengthwise direction duringrecording and reproduction.

The magnetic recording medium according to one embodiment is preferablyused in a library device, which is a large-scale magnetic tape dataarchive device. Also, the magnetic recording medium according to oneembodiment is configured to be capable of recording a signal having ashortest recording wavelength of 75 nm or less for example, and is usedin a recording and reproduction device having a shortest recordingwavelength of 75 nm or less. The above recording and reproduction deviceis provided with a ring head as the head for recording, and uses thering head to record a signal having a shortest recording wavelength of75 nm or less to the magnetic recording medium.

(Coercive force Hc)

A ratio R (=(Hc(50)/Hc(25))×100) of a coercive force Hc(50) measured inthe thickness direction (perpendicular direction) of the magneticrecording medium at 50° C. and a coercive force Hc(25) measured in thethickness direction of the magnetic recording medium at 25° C. is 95% orgreater, preferably 96% or greater, more preferably 97% or greater, andeven more preferably 98% or greater. If the above ratio R is less than95%, the temperature dependence of the coercive force Hc becomes large,and there is a possibility that SNR degradation in a high-temperatureenvironment will become difficult to suppress.

The coercive force Hc(25) measured in the thickness direction(perpendicular direction) of the magnetic recording medium at 25° C. ispreferably 200 kA/m or greater, more preferably 200 kA/m or greater and340 kA/m or less, even more preferably 220 kA/m or greater and 320 kA/mor less, and particularly preferably 230 kA/m or greater and 300 kA/m orless. If the coercive force Hc(25) is less than 200 kA/m, there is apossibility that the above ratio R will be less than 95%. Consequently,the temperature dependence of the coercive force Hc becomes large, andthere is a possibility that SNR degradation in a high-temperatureenvironment will become difficult to suppress. On the other hand, if thecoercive force Hc(25) is 340 kA/m or less, a favorable SNR at 25° C. canbe obtained.

The coercive force Hc(50) measured in the thickness direction of themagnetic recording medium at 50° C. is preferably 190 kA/m or greater,more preferably 190 kA/m or greater and 310 kA/m or less, even morepreferably 200 kA/m or greater and 300 kA/m or less, and particularlypreferably 210 kA/m or greater and 280 kA/m or less. If the coerciveforce Hc(50) is 190 kA/m or greater, there is a possibility that theabove ratio R will be less than 95%. Consequently, the temperaturedependence of the coercive force Hc becomes large, and there is apossibility that SNR degradation in a high-temperature environment willbecome difficult to suppress. On the other hand, if the coercive forceHc is 310 kA/m or less, a favorable SNR at 25° C. can be obtained.

The above coercive force Hc (25) is obtained as follows. First, ameasurement sample is cut out from the elongated magnetic recordingmedium, and a vibrating sample magnetometer (VSM) is used to measure theM-H loop of the entire measurement sample in the thickness direction ofthe measurement sample (the thickness direction of the magneticrecording medium). Next, acetone, ethanol, or the like is used to wipeoff the coating film (such as the foundation layer 12 and the recordinglayer 13) leaving only the substrate 11 to use for backgroundcorrection, and the VSM is used to measure the M-H loop of the substrate11 in the thickness direction of the substrate 11 (the thicknessdirection of the magnetic recording medium). After that, the M-H loop ofthe substrate 11 is subtracted from the M-H loop of the entiremeasurement sample to obtain a background-corrected M-H loop. Thecoercive force Hc is computed from the obtained M-H loop. Note that bothof the measurements of the M-H loop above are assumed to be performed at25° C. Also, it is assumed that “diamagnetic field correction” is notperformed when measuring the M-H loop in the thickness direction(perpendicular direction) of the magnetic recording medium.

The coercive force Hc(50) above is computed similarly to the method ofmeasuring the coercive force Hc(25) above, except that the M-H loops ofthe measurement sample and the substrate 11 are both measured at 50° C.

(Squareness Ratios S1 and S2)

The squareness ratio S1 measured in the transport direction (lengthwisedirection) of the magnetic recording medium is preferably 30% or less,more preferably 25% or less, and even more preferably 20% or less. Ifthe squareness ratio S1 exceeds 30%, the perpendicular orientation ofthe recording layer 13 becomes lower, and there is a possibility thatthe above ratio R will be less than 95%. Consequently, the temperaturedependence of the coercive force Hc becomes large, and there is apossibility that SNR degradation in a high-temperature environment willbecome difficult to suppress.

The squareness ratio S2 measured in the width direction of the magneticrecording medium is preferably 30% or less, more preferably 25% or less,and still more preferably 20% or less. If the squareness ratio S2exceeds 30%, the perpendicular orientation of the recording layer 13becomes lower, and there is a possibility that the above ratio R will beless than 95%. Consequently, the temperature dependence of the coerciveforce Hc becomes large, and there is a possibility that SNR degradationin a high-temperature environment will become difficult to suppress.

The squareness ratio S measured in an arbitrary in-plane direction ofthe magnetic recording medium is preferably 30% or less, more preferably25% or less, and still more preferably 20% or less. If the squarenessratio S exceeds 30%, the perpendicular orientation of the recordinglayer 13 becomes lower, and there is a possibility that the above ratioR will be less than 95%. Consequently, the temperature dependence of thecoercive force Hc becomes large, and there is a possibility that SNRdegradation in a high-temperature environment will become difficult tosuppress.

The above squareness ratio S1 is obtained as follows. First, ameasurement sample is cut out from the elongated magnetic recordingmedium, and a VSM is used to measure the M-H loop of the entiremeasurement sample corresponding to the transport direction (lengthwisedirection) of the magnetic recording medium. Next, acetone, ethanol, orthe like is used to wipe off the coating film (such as the foundationlayer 12 and the recording layer 13) leaving only the substrate 11 touse for background correction, and the VSM is used to measure the M-Hloop of the substrate 11 corresponding to the transport direction of thesubstrate 11 (the transport direction of the magnetic recording medium).After that, the M-H loop of the substrate 11 is subtracted from the M-Hloop of the entire measurement sample to obtain a background-correctedM-H loop. The saturation magnetization Ms (emu) and the residualmagnetization Mr (emu) of the obtained M-H loop are substituted into thefollowing formula to calculate the squareness ratio S1 (%). Note thatboth of the measurements of the M-H loop above are assumed to beperformed at 25° C.Squareness ratio S1(%)=(Mr/Ms)×100

The squareness ratio S2 above is obtained similarly to the method ofmeasuring the squareness ratio S1 above, except that the M-H loops ofthe entire measurement sample and the substrate 11 corresponding to thewidth direction of the magnetic recording medium are measured.

The squareness ratio S above is obtained similarly to the method ofmeasuring the squareness ratio S1 above, except that the M-H loops ofthe entire measurement sample and the substrate 11 corresponding to thearbitrary in-plane direction of the magnetic recording medium aremeasured.

(Activation Volume V_(act))

The activation volume V_(act) is preferably 5000 nm³ or less, morepreferably 4000 nm³ or less, and even more preferably 3000 nm³ or less.If the activation volume V_(act) is 5000 nm³ or less, it is possible tomake a steep bit inversion region, and a favorable SNR can be obtained.

The above activation volume V_(act) is computed according to thefollowing formula derived by Street & Woolley:V _(act) (nm³)=k _(g) ×T×X _(irr)/(μ₀ ×Ms×S)

(where k_(B) is the Boltzmann constant (1.38×10⁻²³ J/K), T is thetemperature (K), X_(irr) is the irreversible susceptibility, μ₀ is themagnetic permeability in a vacuum, S is a magnetic viscositycoefficient, and Ms is the saturation magnetization (emu/cm³)).

The irreversible susceptibility X_(irr), the saturation magnetizationMs, and the magnetic viscosity coefficient S substituted into the aboveformula are computed as follows using the VSM. Note that the directionof measurement by the VSM is assumed to be the thickness direction(perpendicular direction) of the magnetic recording medium. Also, themeasurement by the VSM is assumed to be performed at 25° C. with respectto a measurement sample cut out from the elongated magnetic recordingmedium. Also, it is assumed that “diamagnetic field correction” is notperformed when measuring the M-H loop in the thickness direction(perpendicular direction) of the magnetic recording medium.

(Irreversible Susceptibility X_(irr))

The irreversible susceptibility X_(irr) is defined to be the slope nearthe residual coercive force Hr in the slope of the residualmagnetization curve (DCD curve). First, a magnetic field of −1193 kA/m(15 kOe) is applied to the entire magnetic recording medium, and themagnetic field is returned to zero to create a residual magnetizationstate. After that, a magnetic field of approximately 15.9 kA/m (200 Oe)is applied in the reverse direction and returned to zero again, and theamount of residual magnetization is measured. Similarly after that,measurement is performed repeatedly in which a magnetic field that isanother 15.9 kA/m greater than the previously applied magnetic field isapplied and returned to zero, and the amounts of residual magnetizationwith respect to the applied magnetic fields are plotted to measure theDCD curve. From the obtained DCD curve, the point at which the amount ofmagnetization becomes zero is treated as the residual coercive force Hr,and furthermore the derivative of the DCD curve is taken to compute theslope of the DCD curve for each magnetic field. In this slope of the DCDcurve, the slope near the residual coercive force Hr becomes X_(irr).

(Saturation Magnetization Ms)

First, the M-H loop of the entire magnetic recording medium (measurementsample) is measured in the thickness direction of the magnetic recordingmedium. Next, acetone, ethanol, and the like are used to wipe off thecoating film (such as the foundation layer 12 and the recording layer13) leaving only the substrate 11 to use for background correction, andthe M-H loop of the substrate 11 is measured similarly in the thicknessdirection. After that, the M-H loop of the substrate 11 is subtractedfrom the M-H loop of the entire magnetic recording medium to obtain abackground-corrected M-H loop. From the value of the saturationmagnetization Ms (emu) of the obtained M-H loop and the volume (cm³) ofthe recording layer 13 inside the measurement sample, Ms (emu/cm³) iscomputed. Note that the volume of the recording layer 13 is computed bymultiplying the average thickness of the recording layer 13 by the areaof the measurement sample. The method of computing the average thicknessof the recording layer 13 needed to compute the volume of the recordinglayer 13 will be described later.

(Magnetic Viscosity Coefficient S)

First, a magnetic field of −1193 kA/m (15 kOe) is applied to the entiremagnetic recording medium (measurement sample), and the magnetic fieldis returned to zero to create a residual magnetization state. Afterthat, a magnetic field with substantially the same value of the residualcoercive force Hr obtained from the DCD curve is applied in the reversedirection. The amount of magnetization is measured continually at fixedtime intervals for 1000 seconds in the state with the magnetic fieldapplied. The relationship between the time t and the amount ofmagnetization M(t) obtained in this way is checked against the followingformula to compute the magnetic viscosity coefficient S:M(t)=M0+S×ln(t)

(where M(t) is the amount of magnetization at the time t, M0 is aninitial amount of magnetization, S is the magnetic viscositycoefficient, and ln(t) is the natural logarithm of the time).

(Substrate)

The substrate 11 that acts as a supporting medium is a flexibleelongated non-magnetic substrate. The non-magnetic substrate is a film,and the thickness of the film is 3 μm or greater and 8 μm or less, forexample. For the material of the substrate 11, for example, a polyestersuch as polyethylene terephthalate, a polyolefin such as polyethylene,polypropylene, a cellulose derivative such as cellulose triacetate,cellulose diacetate, or cellulose butyrate, a vinyl resin such aspolyvinyl chloride or polyvinylidene chloride, a plastic such aspolycarbonate, polyimide, or polyamide-imide, a light metal such as analuminum alloy or a titanium alloy, a ceramic such as alumina glass, orthe like can be used.

(Recording Layer)

The recording layer 13 is also referred to as a “perpendicular recordinglayer”, and contains a magnetic powder, a binding agent, and conductingparticles, for example. The recording layer 13 may additionally containadditives such as a lubricant, an abrasive, and an anticorrosive asnecessary.

(Magnetic Powder)

The magnetic powder includes a powder of nanoparticles containing ε ironoxide (hereinafter referred to as “ε iron oxide particles”). The ε ironoxide particles can obtain high holding force even with fine particles.The ε iron oxide particles are spherical or substantially spherical, oralternatively cubic or substantially cubic. Because the ε iron oxideparticles have a shape like the above, in the case of using the ε ironoxide particles as magnetic particles, the contact area betweenparticles with each other in the thickness direction of the medium isreduced compared to the case of using hexagonal plate-like bariumferrite particles as the magnetic particles, and the aggregation ofparticles with each other can be restrained. Consequently, thedispersiveness of the magnetic powder can be raised and a more favorableSNR can be obtained.

Each ε iron oxide particle has a core shell structure. Specifically, asillustrated in FIG. 2, each ε iron oxide particle is provided with acore part 21 and a dual-layer shell part 22 provided on thecircumference of the core part 21. The dual-layer shell part 22 isprovided with a first shell part 22 a provided on the core part 21 and asecond shell part 22 b provided on the first shell part 22 a.

The core part 21 contains ε iron oxide. The ε iron oxide contained inthe core part 21 preferably takes ε-Fe₂O₃ as its primary phase, and morepreferably includes single-phase ε-Fe₂O₃.

The first shell part 22 a that is a soft magnetic layer covers at leasta part of the circumference of the core part 21. Specifically, the firstshell part 22 a may cover the circumference of the core part 21partially or cover the entire circumference of the core part 21. Theexchange-coupling between the core part 21 and the first shell part 22 ais taken to be sufficient, and from the perspective of improving themagnetic properties, the exchange-coupling preferably covers the entiresurface of the core part 21.

The first shell part 22 a is also referred to as a “soft magneticlayer”, and contains a soft magnetic material such as α-Fe, Ni—Fe alloy,or Fe—Si—Al alloy, for example. The α-Fe may also be obtained byreducing the ε iron oxide contained in the core part 21.

The second shell part 22 b is an oxide film that acts as an antioxidantlayer. The second shell part 22 b contains a iron oxide, aluminum oxide,or silicon oxide. The α oxide contains at least one type of iron oxidefrom among Fe₃O₄, Fe₂O₃, and FeO, for example. In the case in which thefirst shell part 22 a contains α-Fe (a soft magnetic material), the αiron oxide may also be obtained by oxidizing the α-Fe contained in thefirst shell part 22 a.

By having each ε iron oxide particle include the first shell part 22 aas described above, the coercive force Hc of the core part 21 by itselfcan retain a large value to ensure thermostability, while the coerciveforce Hc for the ε iron oxide particle (core shell particle) as a wholecan be adjusted to a coercive force Hc suited to recording. Also, byhaving each ε iron oxide particle include the second shell part 22 b asdescribed above, in and before the process of manufacturing the magneticrecording medium, the ε iron oxide particle is exposed to the air toproduce rust and the like on the particle surface, and thereby alowering of the properties of the ε iron oxide particle can berestrained. Consequently, degradation in the properties of the magneticrecording medium can be restrained.

The average particle size (average maximum particle size) D of themagnetic powder is preferably 22 nm or less, more preferably 8 nm orgreater and 22 nm or less, and even more preferably 12 nm or greater and22 nm or less.

The average particle size D of the magnetic powder above is obtained asfollows. First, the magnetic recording medium to be measured isprocessed by the focused ion beam (FIB) method or the like to fabricatea thin section, and cross-sectional observation of the thin section isperformed with a transmission electron microscope (TEM). Next, 500 εiron oxide particles are chosen randomly from the taken TEM photograph,a maximum particle size d_(max) of each of the particles is measured,and a particle distribution of the maximum particle size d_(max) of themagnetic powder is obtained. Herein, the “maximum particle size d_(max)”means what is also referred to as the maximum Feret diameter, andspecifically refers to the maximum from among the distances between twoparallel lines drawn from every angle tangent to the contours of the εiron oxide particles. After that, the median diameter (50% diameter,D50) of the maximum particle size d_(max) is computed from the particledistribution of the computed maximum particle size d_(max), and this istaken to be the average particle size (average maximum particle size) D.

The average thickness of the recording layer 13 is preferably 30 nm orgreater and 120 nm or less, more preferably 40 nm or greater and 100 nmor less, even more preferably 40 nm or greater and 80 nm or less, andmost preferably 40 nm or greater and 70 nm or less.

The average thickness [nm] of the recording layer 13 is computed asfollows. First, the magnetic recording medium to be measured isprocessed by the FIB method or the like to fabricate a thin sectionhaving a principal plane parallel to the width direction of the magneticrecording medium, and cross-sectional observation of the thin section isperformed with a TEM. Observation is performed preferably with anobservation magnification of at least 100,000× or greater to make itpossible to observe the thickness of the recording layer 13 clearly.Cross-sectional TEM observation is performed at a position every 100 min the lengthwise direction (transport direction) of the magnetic, for atotal of five locations. The thickness of the recording layer 13 isobserved at 50 points uniformly per field of view, and the thicknessesof all five fields of view are simply averaged (arithmetic mean) tocompute the average thickness [nm] of the recording layer 13.

(Binding Agent)

For the binding agent, a resin with a structure obtained by impartingcross-linking reactions to a polyurethane resin, a vinyl chloride resin,or the like is preferable. However, the binding agent is not limited tothese, and may be combined appropriately with another resin according tothe physical properties demanded of the magnetic recording medium andthe like. Ordinarily, the resin to combine is not particularly limitedinsofar as the resin is typically used in a coating-type magneticrecording medium.

Examples include polyvinyl chloride, polyvinyl acetate, vinylchloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloridecopolymers, vinyl chloride-acrylonitrile copolymers, acrylic acidester-acrylonitrile copolymers, acrylic acid ester-vinylchloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrilecopolymers, acrylic acid ester-acrylonitrile copolymers, acrylic acidester-vinylidene chloride copolymers, methacrylic acid ester-vinylidenechloride copolymers, methacrylic acid ester-vinyl chloride copolymers,methacrylic acid ester-ethylene copolymers, polyvinyl fluoride,vinylidene chloride-acrylonitrile copolymers, acrylonitrile-butadienecopolymers, polyamide resin, polyvinyl butyral, cellulose derivatives(cellulose acetate butyrate, cellulose diacetate, cellulose triacetate,cellulose propionate, nitrocellulose), styrene-butadiene copolymers,polyester resin, amino resin, and synthetic rubber.

Also, examples of thermosetting resins or reactive resins includephenolic resin, epoxy resin, urea resin, melamine resin, alkyd resin,silicone resin, polyamine resin, urea formaldehyde resin, and the like.

Also, for the purpose of improving the dispersiveness of the magneticpowder, a polar functional group such as —SO₃M, —OSO₃M, —COOM, orP═O(OM)₂ may also be introduced into each binding agent described above.Herein, “M” in the formulas is a hydrogen atom or an alkali metal suchas lithium, potassium, or sodium.

Furthermore, the polar functional group may be one with a side chainhaving an end group of —NR1R2 or —NR1R2R3⁺X⁻, or one with a main chainof >NR1R2⁺X⁻. Herein, “R1”, “R2”, and “R3” in the formulas is a hydrogenatom or a hydrocarbon group, and X⁻ is an ion of a halogen element suchas fluorine, chlorine, bromine, or iodine, or is an inorganic or organicion. Also, the polar functional group may also be —OH, —SH, —CN, anepoxy group, or the like.

(Additives)

The recording layer 13 additionally may contain aluminum oxide (α,β, orγ alumina), chrome oxide, silicon oxide, diamond, garnet, emery, boronnitride, titanium carbide, silicon carbide, titanium carbide, titaniumoxide (rutile or anatase titanium oxide), or the like as non-magneticreinforcing particles.

(Foundation Layer)

The foundation layer 12 is a non-magnetic layer containing anon-magnetic powder and a binding agent as its main components. Asnecessary, the foundation layer 12 may additionally contain at least onetype of additive from among conducting particles, a lubricant, ahardener, an anticorrosive, and the like.

(Average Thickness of Foundation Layer)

The average thickness of the foundation layer 12 is preferably 0.6 μm orgreater and 2.0 μm or less, and more preferably 0.8 μm or greater and1.4 μm or less. Note that the method of computing the average thicknessof the foundation layer 12 above is similar to the method of computingthe average thickness of the recording layer 13.

(Non-Magnetic Powder)

The non-magnetic powder may be an inorganic substance or an organicsubstance. Also, the non-magnetic powder may be carbon black or thelike. Examples of inorganic substances include metal, metal oxide, metalcarbonate, metal sulfate, metal nitride, metal carbide, metal sulfide,and the like. The shape of the non-magnetic powder may be any of variousshapes such as needle-like, spherical, cubic, and plate-like, forexample, but is not limited thereto.

(Binding Agent)

The binding agent is similar to the recording layer 13 described above.

[Method of Manufacturing Magnetic Recording Medium]

Next, one example of a method of manufacturing the magnetic recordingmedium having the configuration described above will be described.First, a foundation layer-forming coating is prepared by kneading anddispersing the non-magnetic powder, the binding agent, and the like intoa solvent. Next, a recording layer-forming coating is prepared bykneading and dispersing the magnetic powder, the binding agent, and thelike into a solvent. In the preparation of the recording layer-formingcoating and the foundation layer-forming coating, the followingsolvents, dispersing devices, and kneading devices can be used, forexample.

Examples of solvents used in the coating preparation described aboveinclude ketone solvents such as acetone, methyl ethyl ketone, methylisobutyl ketone, and cyclohexanone, alcohol solvents such as methanol,ethanol, and propanol, ester solvents such as methyl acetate, ethylacetate, butyl acetate, propyl acetate, ethyl lactate, and ethyleneglycol acetate, ether solvents such as diethylene glycol dimethyl ether,2-ethoxyethanol, tetrahydrofuran, and dioxane, aromatic hydrocarbonsolvents such as benzene, toluene, and xylene, halogenated hydrocarbonsolvents such as methylene chloride, ethylene chloride, carbontetrachloride, chloroform, and chlorobenzene, and the like. These may beused alone or by being mixed appropriately.

As the kneading device used in the coating preparation described above,a kneading device such as a continuous two-axis kneading machine, acontinuous two-axis kneading machine capable of multi-stage dilution, akneader, a pressure kneader, or a roll kneader can be used, for example,but the kneading device is not particularly limited to these devices.Also, as the dispersing device used in the coating preparation describedabove, a dispersing device such as a roll mill, a ball mill, ahorizontal sand mill, a vertical sand mill, a spike mill, a pin mill, atower mill, a pearl mill (such as “DCP Mill” manufactured by the EirichGroup), a homogenizer, or an ultrasonic dispersion machine can be used,but the dispersing device is not particularly limited to these devices.

Next, by applying the foundation layer-forming coating to one of theprincipal planes of the substrate 11 and drying, the foundation layer 12is formed. Next, by applying the recording layer-forming coating on thefoundation layer 12 and drying, the recording layer 13 is formed on thefoundation layer 12. Note that while drying, the magnetic field of themagnetic powder is oriented in the thickness direction of the substrate11 with a solenoid coil, for example. After forming the recording layer13, the protective layer and the lubricant layer may be formed on therecording layer 13, and the backcoat layer 14 may also be formed on theother principal plane of the substrate 11 as necessary.

After that, the substrate 11 having the foundation layer 12 and therecording layer 13 formed thereon is rewound around a large-diametercore, and a hardening treatment is performed. Lastly, the substrate 11having the foundation layer 12 and the recording layer 13 formed thereonis subjected to a calender process and then cut to a predeterminedwidth. By the above, the intended magnetic recording medium is obtained.

Effects

The magnetic recording medium according to one embodiment of the presenttechnology is provided with the recording layer 13 containing a powderof particles containing ε iron oxide. Also, the ratio((Hc(50)/Hc(25))×100) of the coercive force Hc(50) measured in thethickness direction of the magnetic recording medium at 50° C. and thecoercive force Hc(25) measured in the thickness direction of themagnetic recording medium at 25° C. is 95% or greater, the coerciveforce Hc(25) is 200 kA/m or greater, and the squareness ratio S1measured in the transport direction of the magnetic recording medium is30% or less. With this arrangement, the temperature dependence of thecoercive force Hc can be reduced, and SNR degradation in ahigh-temperature environment can be suppressed.

Also, in the case in which the ratio ((Hc(50)/Hc(25))×100) of thecoercive force Hc(50) measured in the thickness direction of themagnetic recording medium at 50° C. and the coercive force Hc(25)measured in the thickness direction of the magnetic recording medium at25° C. is 95% or greater, the squareness ratio S1 measured in thetransport direction of the magnetic recording medium is 30% or less, thecoercive force Hc(25) is 200 kA/m or greater and 340 kA/m or less, andthe activation volume is 5000 nm³ or less, SNR degradation in ahigh-temperature environment can be suppressed, while in addition, afavorable SNR can be obtained.

[Modifications]

(Modification 1)

The foregoing embodiment describes a case in which each ε iron oxideparticle has the dual-layer shell part 22, but as illustrated in FIG. 3,each ε iron oxide particle may have a single-layer shell part 23. Inthis case, the shell part 23 has a configuration similar to the firstshell part 22 a. However, from the perspective of restrainingdegradation in the properties of the ε iron oxide particles, it ispreferable for each ε iron oxide particle to have the dual-layer shellpart 22 like in the foregoing embodiment.

(Modification 2)

As illustrated in FIG. 4, the magnetic recording medium additionally maybe provided with a reinforcement layer 15 provided on the otherprincipal plane on the backcoat layer 14 side (hereinafter referred toas the “back face”) from among the two principal planes of the substrate11. In this case, the backcoat layer 14 is provided on the reinforcementlayer 15.

Note that the reinforcement layer 15 may be provided on either of thetwo principal planes of the substrate 11, and the reinforcement layer 15may also be provided on the principal plane on the recording layer 13side (hereinafter referred to as the “front face”) from among the twoprincipal planes of the substrate 11. In this case, the foundation layer12 is provided on the reinforcement layer 15.

The reinforcement layer 15 is for raising the mechanical strength of themagnetic recording medium and obtaining excellent dimensional stability.The reinforcement layer 15 contains at least one from among metals andmetal compounds, for example. Herein, metals are defined to includesemimetals. The metal is at least one of aluminum or copper, and ispreferably copper, for example. This is because copper is inexpensivewith a relatively low vapor pressure, thereby making it possible to formthe reinforcement layer 15 inexpensively. The metal compound is ametallic oxide, for example. The metallic oxide is at least one fromamong aluminum oxide, copper oxide, and silicon oxide, and is preferablycopper oxide, for example. This is because forming the reinforcementlayer 15 inexpensively by vapor deposition or the like is possible. Forexample, the reinforcement layer 15 may be a vapor-deposited film formedby vacuum oblique angle vapor deposition, or a sputtered film formed bysputtering.

It is preferable for the reinforcement layer 15 to have a laminatestructure of two or more layers. As the thickness of the reinforcementlayer 15 is increased, expansion and contraction of the substrate 11 inresponse to external forces can be suppressed further. However, in thecase of forming the reinforcement layer 15 using a vacuum thin-filmmanufacturing technique such as vapor deposition or sputtering, as thethickness of the reinforcement layer 15 is increased as above, there isa possibility of gaps in the reinforcement layer 15 occurring morereadily. By causing the reinforcement layer 15 to have a laminatestructure of two or more layers as above, the occurrence of gaps in thereinforcement layer 15 when forming the reinforcement layer 15 using avacuum thin-film manufacturing technique can be suppressed, and thecompactness of the reinforcement layer 15 can be improved. Consequently,because the moisture vapor transmission rate of the reinforcement layer15 can be reduced, swelling of the substrate 11 can be restrainedfurther, and the dimensional stability of the magnetic recording mediumcan be improved further. In the case in which the reinforcement layer 15has a laminate structure of two or more layers, the material of eachlayer may be the same or different.

The average thickness of the reinforcement layer 15 is preferably 150 nmor greater and 500 nm or less. If the average thickness of thereinforcement layer 15 is 150 nm or greater, favorable functionality asthe reinforcement layer 15 (that is, favorable dimensional stability ofthe magnetic recording medium) is obtained.

On the other hand, sufficient functionality as the reinforcement layer15 can be obtained without having to increase the average thickness ofthe reinforcement layer 15 past 500 nm. Note that the average thicknessof the reinforcement layer 15 above is computed similarly to the methodof computing the average thickness of the recording layer 13 describedearlier.

In the case in which the magnetic recording medium has the reinforcementlayer 15, the Young's modulus in the lengthwise direction of theelongated magnetic recording medium is preferably 7 GPa or greater and14 GPa or less. If Young's modulus is 7 GPa or greater, a favorablemagnetic head hit can be obtained, and in addition, edge damage can berestrained. On the other hand, if Young's modulus is 14 GPa or less, afavorable magnetic head hit can be obtained.

Also, the coefficient of humidity expansion of the magnetic recordingmedium is preferably 0.5 ppm/% RH or greater and 4 ppm/% RH or less. Ifthe coefficient of humidity expansion is in the above range, thedimensional stability of the magnetic recording medium can be improvedfurther.

(Modification 3)

As illustrated in FIG. 5, the magnetic recording medium additionally maybe provided with a cupping suppression layer 16 provided on thereinforcement layer 15. Note that in the case in which the foundationlayer 12 and the cupping suppression layer 16 are provided on the backface side of the substrate 11, the backcoat layer 14 is provided on thecupping suppression layer 16. On the other hand, in the case in whichthe foundation layer 12 and the cupping suppression layer 16 areprovided on the front face side of the substrate 11, the foundationlayer 12 is provided on the cupping suppression layer 16.

The cupping suppression layer 16 is for suppressing cupping that occursdue to formation of the reinforcement layer 15 on the substrate 11.Herein, cupping means curvature occurring in the width direction of theelongated substrate 11. In the reinforcement layer 15, tensile stressacts as an internal stress, or in other words, stress works to curve theprincipal plane side on which the reinforcement layer 15 is providedfrom among the two principal planes of the substrate 11 in the widthdirection and in a concave shape. In contrast, in the cuppingsuppression layer 16, compressive stress acts as an internal stress, orin other words, stress works to curve the principal plane side on whichthe cupping suppression layer 16 is provided from among the twoprincipal planes of the substrate 11 in the width direction and in aconvex shape. For this reason, the internal stresses of thereinforcement layer 15 and the cupping suppression layer 16 cancel eachother out, and the occurrence of cupping in the magnetic recordingmedium can be suppressed. Consequently, it is possible to provide amagnetic recording medium that can keep the state of contact between themagnetic head and the magnetic recording medium in a favorable state,while also excellent off-track characteristics with high dimensionalstability in the track width direction.

The cupping suppression layer 16 is a carbon thin film, for example. Thecarbon thin film is preferably a hard carbon thin film containingdiamond-like carbon (hereinafter referred to as “DLC”). For example, thecupping suppression layer 16 may be a chemical vapor deposition (CVD)film formed by CVD, or a sputtered film formed by sputtering.

It is preferable for the cupping suppression layer 16 to have a laminatestructure of two or more layers. This is because the dimensionalstability of the magnetic recording medium can be improved further. Notethat the principle of the above is similar to the case of causing thereinforcement layer 15 to have a laminate structure of two or morelayers. In the case in which the cupping suppression layer 16 has alaminate structure of two or more layers, the material of each layer maybe the same or different.

The average thickness of the cupping suppression layer 16 is preferably10 nm or greater and 200 nm or less. If the average thickness of thecupping suppression layer 16 is less than 10 nm, there is a possibilityof the compressive stress of the cupping suppression layer 16 becomingtoo small. On the other hand, if the average thickness of the cuppingsuppression layer 16 exceeds 200 nm, there is a possibility of thecompressive stress of the cupping suppression layer 16 becoming toolarge. Note that the average thickness of the cupping suppression layer16 is computed similarly to the method of computing the averagethickness of the recording layer 13 described earlier.

(Modification 4)

As illustrated in FIG. 6, the magnetic recording medium additionally maybe provided with a first reinforcement layer 17 provided on the frontface of the substrate 11, a second reinforcement layer 18 provided onthe back face of the substrate 11, and an adhesion suppression layer 19provided on the second reinforcement layer 18. In this case, thebackcoat layer 14 is provided on the adhesion suppression layer 19. Thesubstrate 11, the first reinforcement layer 17, the second reinforcementlayer 18, and the adhesion suppression layer 19 form a laminate 10.

Note that it is sufficient for the adhesion suppression layer 19 to beprovided on one of the first and second reinforcement layers 17 and 18,and the adhesion suppression layer 19 may also be provided on the firstreinforcement layer 17. In this case, the foundation layer 12 isprovided on the adhesion suppression layer 19. In this case, if theadhesion suppression layer 19 is a carbon thin film, it is preferable toimprove the wettability of the surface of the adhesion suppression layer19 by a surface reforming treatment. This is because the coatingproperties of the foundation layer-forming coating with respect to thecarbon thin film can be improved.

The first and second reinforcement layers 17 and 18 are for raising themechanical strength of the magnetic recording medium and obtainingexcellent dimensional stability. As for the materials of the first andsecond reinforcement layers 17 and 18, materials similar to those in thereinforcement layer 15 of Modification 2 can be given as examples. Notethat the first and second reinforcement layers 17 and 18 may be the samematerial or different materials. It is preferable for each of the firstand second reinforcement layers 17 and 18 to have a laminate structureof two or more layers. This is because the dimensional stability of themagnetic recording medium can be improved further. Note that theprinciple of the above is similar to the case of causing thereinforcement layer 15 to have a laminate structure of two or morelayers in Modification 2.

The average thickness of the first and second reinforcement layers 17and 18 is preferably 75 nm or greater and 300 nm or less. If the averagethickness of the first and second reinforcement layers 17 and 18 is 75nm or greater, favorable functionality as the first and secondreinforcement layers 17 and 18 (that is, favorable dimensional stabilityof the magnetic recording medium) is obtained. On the other hand, if theaverage thickness of the first and second reinforcement layers 17 and 18is increased past 300 nm, there is a possibility of the magneticrecording medium becoming thick. Also, sufficient functionality as thefirst and second reinforcement layers 17 and 18 can be obtained withouthaving to increase the average thickness of the first and secondreinforcement layers 17 and 18 past 300 nm. Note that the averagethickness of the first and second reinforcement layers 17 and 18 aboveis computed similarly to the method of computing the average thicknessof the recording layer 13 described earlier.

In the first and second reinforcement layers 17 and 18, tensile stressworks as an internal stress. Specifically, in the first reinforcementlayer 17, stress works to curve the front face side of the substrate 11in the width direction and in a concave shape, while in the secondreinforcement layer 18, stress works to curve the back face side of thesubstrate 11 in the width direction and in a concave shape.Consequently, the internal stresses of the first and secondreinforcement layers 17 and 18 cancel each other out, and the occurrenceof cupping in the magnetic recording medium can be suppressed. Herein,cupping means curvature occurring in the width direction of theelongated substrate 11.

The average thickness of the first and second reinforcement layers 17and 18 may be the same or different, but preferably is the same orsubstantially the same. This is because the internal stresses (tensilestresses) in the first and second reinforcement layers 17 and 18provided on either side of the substrate 11 become the same orsubstantially the same, and the occurrence of cupping can be suppressedfurther. Herein, the average thickness of the first and secondreinforcement layers 17 and 18 being substantially the same means thatthe difference in average thickness between the first and secondreinforcement layers 17 and 18 is within 5 nm.

The adhesion suppression layer 19 is for suppressing metallic adhesionand sticking by the first and second reinforcement layers 17 and 18 inthe case in which the laminate 10 is wound into a roll. The adhesionsuppression layer 19 may be electrically conductive or insulating. Theadhesion suppression layer 19 may be a layer in which compressive stressacts as an internal stress (in other words, stress works to curve theplane side on which the adhesion suppression layer 19 is provided fromamong the two principal planes of the substrate 11 in the widthdirection and in a convex shape), or a layer in which tensile stressacts as an internal stress (in other words, stress works to curve theplane side on which the adhesion suppression layer 19 is provided fromamong the substrate 11 in the width direction and in a concave shape).

In the case in which the tensile stresses (internal stresses) of thefirst and second reinforcement layers 17 and 18 are different, theadhesion suppression layer 19 having a compressive stress that works asan internal stress may be provided on the reinforcement layer having thelarger tensile stress from among the first and second reinforcementlayers 17 and 18. This is because the tensile stress that is notcanceled out due to the difference in the tensile stresses of the firstand second reinforcement layers 17 and 18 can be canceled out by thecompressive stress of the adhesion suppression layer 19. Also, theadhesion suppression layer 19 having a tensile stress that works as aninternal stress may be provided on the reinforcement layer having thesmaller tensile stress from among the first and second reinforcementlayers 17 and 18. This is because the compressive stress produced by thedifference in the tensile stresses of the first and second reinforcementlayers 17 and 18 can be canceled out by the tensile stress of theadhesion suppression layer 19.

The average thickness of the adhesion suppression layer 19 is preferably1 nm or greater and 100 nm or less, more preferably 2 nm or greater and25 nm or less, and even more preferably 2 nm or greater and 20 nm orless. If the average thickness of the adhesion suppression layer 19 is 1nm or greater, it is possible to keep the average thickness of theadhesion suppression layer 19 from becoming too thin, and a reduction infunctionality as the adhesion suppression layer 19 can be suppressed. Onthe other hand, if the average thickness of the adhesion suppressionlayer 19 is 100 nm or less, it is possible to keep the average thicknessof the adhesion suppression layer 19 from becoming too thick, or inother words, keep the internal stress of the adhesion suppression layer19 from becoming too large. The average thickness of the adhesionsuppression layer 19 is computed similarly to the method of computingthe average thickness of the recording layer 13 described earlier.

In the case in which an average thickness D2 of the second reinforcementlayer 18 is 75 nm or greater and 300 nm or less, a ratio (D4/D2) of anaverage thickness D4 of the adhesion suppression layer 19 with respectto the average thickness D2 of the second reinforcement layer 18 ispreferably 0.005 or greater and 0.35 or less. If the ratio (D4/D2) is0.005 or less, it is possible to keep the average thickness D4 of theadhesion suppression layer 19 from becoming too thin with respect to theaverage thickness D2 of the second reinforcement layer 18, and areduction in functionality as the adhesion suppression layer 19 can besuppressed. On the other hand, if the ratio (D4/D2) is 0.35 or less, itis possible to keep the average thickness D4 of the adhesion suppressionlayer 19 from becoming too thick with respect to average thickness D2 ofthe second reinforcement layer 18, or in other words, keep thecompressive stress of the adhesion suppression layer 19 from becomingtoo large with respect to the tensile stress of the second reinforcementlayer 18. Consequently, the occurrence of cupping can be suppressedfurther.

The adhesion suppression layer 19 contains at least one from amongcarbon and metallic oxides, for example. It is preferable for theadhesion suppression layer 19 to be a carbon thin film having carbon asits main component or a metallic oxide film having a metallic oxide asits main component. The carbon is preferably diamond-like carbon(hereinafter referred to as “DLC”). The metallic oxide preferablycontains at least one from among aluminum oxide, copper oxide, andcobalt oxide. For example, the adhesion suppression layer 19 may be achemical vapor deposition (CVD) film formed by CVD, or a sputtered filmformed by sputtering.

It is preferable for the adhesion suppression layer 19 to have alaminate structure of two or more layers. This is because thedimensional stability of the magnetic recording medium can be improvedfurther. Note that the principle of the above is similar to the case ofcausing the reinforcement layer 15 to have a laminate structure of twoor more layers in Modification 3. In the case in which the adhesionsuppression layer 19 has a laminate structure of two or more layers, thematerial of each layer may be the same or different.

In the magnetic recording medium having the configuration describedabove, the internal stresses (tensile stresses) of the first and secondreinforcement layers 17 and 18 cancel each other out, and the occurrenceof cupping in the magnetic recording medium can be suppressed.Consequently, it is possible to provide a magnetic recording medium thatcan keep the state of contact between the magnetic head and the magneticrecording medium in a favorable state, while also excellent off-trackcharacteristics with high dimensional stability in the track widthdirection. Also, in the magnetic recording medium manufacturing process,when the laminate 10 is wound into a roll, because the adhesionsuppression layer 19 is interposed between the first and secondreinforcement layers 17 and 18, metallic adhesion of the first andsecond reinforcement layers 17 and 18 can be suppressed.

(Modification 5)

The magnetic recording medium may also be configured to be capable ofrecording a signal having a shortest recording wavelength of 75 nm orless using a recording and reproduction device provided with a recordinghead other than a ring head (such as a single pole type (SPT) recordinghead, for example).

EXAMPLES

Hereinafter, the present technology will be described specifically usingExamples, but the present technology is not limited to these Examplesonly.

In the following Examples 1 to 12 and Comparative Examples 1 to 8, theaverage particle size, the coercive force Hc (25), Hc (50), thesquareness ratios S1 and S2, the activation volume V_(act), the averagethickness of the recording layer, and the average thickness of thefoundation layer were obtained according to the methods described in theforegoing embodiment.

The following particles A1 to A5 and particles B to F were prepared asthe magnetic powder for the recording layer.

[Powders of particles A1 to A5]

Powders of the particles A1 to A5 were fabricated as follows. First, apowder of substantially spherical ε iron oxide nanoparticles (ε-Fe₂O₃crystal particles) was prepared. Next, by subjecting the powder of εiron oxide nanoparticles to a reduction treatment and a gradualoxidation treatment as follows, a powder of core-shell ε iron oxidenanoparticles having a dual-layer shell part was obtained.

(Reduction Treatment)

First, the powder of ε iron oxide nanoparticles were placed on a quartzboat and loaded into a tube furnace. After loading, the tube furnace wasonce displaced with an N₂ atmosphere, and then raised to a predeterminedtemperature. After raising the temperature, a heat treatment at 350° C.was performed while causing 100% H₂ to flow at a flow rate of 100ml/min. With this arrangement, the surface of the ε iron oxidenanoparticles was transformed into reduced α-Fe, and an α-Fe layer wasformed on the surface of the ε iron oxide nanoparticles. At this point,by adjusting the time of the heat treatment (reduction treatment) at350° C. in a range from 0.04 h to 0.45 h as illustrated in Table 1, thethickness of the α-Fe layer was varied within a range from 0.3 nm to 3.5nm. After that, the tube furnace was displaced with an N₂ atmosphereagain and cooled to room temperature. With this arrangement, a powder ofcore-shell ε iron oxide nanoparticles having the α-Fe on the surface wasobtained.

(Gradual Oxidation Treatment)

Next, the tube furnace was heated to a predetermined temperature, and aheat treatment was performed for at 300° C. for 5 minutes while causingN₂ gas containing trace oxygen to flow at a flow rate of 100 ml/min.With this arrangement, the surface of the α-Fe layer was oxidized, andan α-Fe₂O₃ layer was formed on the surface of the α-Fe layer. Afterthat, the tube furnace was displaced with an N₂ atmosphere again andcooled to room temperature. By the above, powders of core-shell ε ironoxide nanoparticles having an α-Fe₂O₃ layer (oxide film) and an α-Felayer (soft magnetic layer) on the surface with an average particle sizeof 20 nm (powders of the particles A1 to A5) were obtained.

[Powders of Particles B]

Powders of the particles B were fabricated as follows. First, a powderof substantially spherical ε iron oxide nanoparticles (ε-Fe₂O₃ crystalparticles) was prepared. Next, by subjecting the powder of ε iron oxidenanoparticles to a reduction treatment and a sputtering process asfollows, a powder of core-shell ε iron oxide nanoparticles having adual-layer shell part was obtained.

(Reduction Treatment)

First, by performing a reduction treatment similar to the powder of theparticles A2, a powder of core-shell ε iron oxide nanoparticles havingan α-Fe layer 2 nm thick on the surface was obtained.

(Sputtering Process)

Next, the particles after the reduction treatment were conveyed into achamber for performing the sputtering process so as not to expose theparticles to the air. Subsequently, by performing the sputtering processusing an Al₂O₃ target while causing the particles to vibrate, a powderof core-shell ε iron oxide nanoparticles having an Al₂O₃ layer (oxidefilm) and an α-Fe layer (soft magnetic layer) on the surface with anaverage particle size of 20 nm (powder of the particles B) was obtained.

[Powders of Particles C]

Powders of the particles C were fabricated as follows. First, a powderof substantially spherical ε iron oxide nanoparticles (ε-Fe₂O₃ crystalparticles) was prepared. Next, by subjecting the powder of ε iron oxidenanoparticles to the sputtering process as follows, a powder ofcore-shell ε iron oxide nanoparticles having a dual-layer shell part wasobtained.

(Sputtering Process (Soft Magnetic Ni—Fe Film Deposition))

First, the ε iron oxide nanoparticles were conveyed into a chamber forperforming the sputtering process. Subsequently, by performing thesputtering process using an Ni—Fe target while causing the particles tovibrate, a powder of core-shell ε iron oxide nanoparticles having anNi—Fe alloy layer on the surface was obtained.

(Sputtering Process (Al₂O₃ Layer))

Next, by performing the sputtering process similar to the powder of theparticles B, a powder of core-shell ε iron oxide nanoparticles having anAl₂O₃ layer (oxide film) and an Ni—Fe alloy layer (soft magnetic layer)on the surface with an average particle size of 20 nm (powder of theparticles C) was obtained.

[Powder of Particles D]

A powder of substantially spherical ε iron oxide nanoparticles (ε-Fe₂O₃crystal particles) with an average particle size of 20 nm was preparedand taken to be the powder of the particles D in an unchanged state,without performing the reduction treatment and the gradual oxidationtreatment.

[Powder of Particles E]

For the powder of the particles E, a powder of A1-doped, substantiallyspherical ε iron oxide nanoparticles with an average particle size of 17nm was prepared.

[Powders of Particles F]

For the powder of the particles F, a powder of Ga-doped, substantiallyspherical ε iron oxide nanoparticles with an average particle size of 17nm was prepared.

The above powders of the particles A1 to A5 and the particles B to F,the powder of barium ferrite (BaFe) particles, and the powder of metalparticles were used to fabricate the magnetic tape of Examples 1 to 12and Comparative Examples 1 to 8 as follows.

Examples 1 to 5, 8 to 10, and Comparative Examples 1 and 2

(Recording Layer-Forming Coating Preparation Step)

The recording layer-forming coating was prepared as follows. First, afirst composition with the formulation below was kneaded in an extruder.Next, the kneaded first composition and a second composition with theformulation below were added to a mixing tank provided with a disperserand premixed. Next, sand mill mixing was additionally performed, afilter treatment was performed, and the recording layer-forming coatingwas prepared.

(First Composition)

Powders of particles A1 to A5 (see Tables 1 and 2): 100 parts by mass

Vinyl chloride resin (cyclohexanone solution 30% by mass): 10 parts bymass

(degree of polymerization 300, Mn=10000, containing OSO₃K=0.07 mmol/g,second-order OH=0.3 mmol/g as a polar group)

Aluminum oxide powder: 5 parts by mass

(α-A1₂O₃, average particle diameter 0.2 μm)

Carbon black: 2 parts by mass

(manufactured by Tokai Carbon Co., Ltd., product name: “SEAST TA”)

(Second Composition)

Vinyl chloride resin: 1.1 parts by mass

(resin solution: resin content 30% by mass, cyclohexanone 70% by mass)

n-Butyl stearate: 2 parts by mass

Methyl ethyl ketone: 121.3 parts by mass

Toluene: 121.3 parts by mass

Cyclohexanone: 60.7 parts by mass

Lastly, as a hardener, 4 parts by mass of polyisocyanate (product name:“Coronate L”, manufactured by Nippon Polyurethane Industry Co., Ltd.) inand 2 parts by mass of myristic acid were added to the recordinglayer-forming coating prepared as described above.

(Foundation Layer-Forming Coating Preparation Step)

The foundation layer-forming coating was prepared as follows. First, athird composition with the formulation below was kneaded in an extruder.Next, the kneaded third composition and the fourth composition with theformulation below were added to the mixing tank provided with adisperser and premixed. Next, sand mill mixing was additionallyperformed, a filter treatment was performed, and the foundationlayer-forming coating was prepared.

(Third Composition)

Needle-like iron oxide powder: 100 parts by mass

(α-Fe₂O₃, average long-axis length 0.15 μm)

Vinyl chloride resin: 55.6 parts by mass

(resin solution: resin content 30% by mass, cyclohexanone 70% by mass)

Carbon black: 10 parts by mass

(Average particle diameter 20 nm)

(Fourth Composition)

Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.): 18.5 partsby mass

n-Butyl stearate: 2 parts by mass

Methyl ethyl ketone: 108.2 parts by mass

Toluene: 108.2 parts by mass

Cyclohexanone: 18.5 parts by mass

Lastly, as a hardener, 4 parts by mass of polyisocyanate (product name:“Coronate L”, manufactured by Nippon Polyurethane Industry Co., Ltd.)and 2 parts by mass of myristic acid were added to the foundationlayer-forming coating prepared as described above.

(Backcoat Layer-Forming Coating Preparation Step)

The backcoat layer-forming coating was prepared as follows. By mixingthe raw materials below in the mixing tank provided with a disperser andperforming a filter treatment, the backcoat layer-forming coating wasprepared.

Carbon black (manufactured by Asahi Carbon Co., Ltd., product name:“#80”): 100 parts by mass

Polyester polyurethane: 100 parts by mass

(manufactured by Nippon Polyurethane Industry Co., Ltd., product name:“N-2304”)

Methyl ethyl ketone: 500 parts by mass

Toluene: 400 parts by mass

Cyclohexanone: 100 parts by mass

(Film Formation Step)

The coatings fabricated as described above were used to form, on apolyethylene naphthalate film (PEN film) 6.2 μm thick which is anon-magnetic supporting medium, a foundation layer with an averagethickness of 1.2 μm and a recording layer with an average thickness of75 nm as follows. First, by applying the foundation layer-formingcoating to the PEN film and drying, the foundation layer was formed onthe PEN film. Next, by applying the recording layer-forming coating tothe foundation layer and drying, the recording layer was formed on thefoundation layer. Note that while drying the recording layer-formingcoating, the magnetic field of the magnetic powder was oriented in thethickness direction of the PEN film with a solenoid coil. Next, acalender process was performed on the PEN film having the foundationlayer and the recording layer formed thereon, and the recording layersurface was smoothed. After that, by applying the backcoat layer-formingcoating in a film 0.6 μm thick to the face on the opposite side from therecording layer and drying, a backcoat layer was formed.

(Cutting Step)

The PEN film having the foundation layer, the recording layer, and thebackcoat layer formed thereon as described above was cut to a width of ½inch (12.65 mm). With this arrangement, magnetic tapes having thecoercive forces Hc (25), Hc (50), the squareness ratios S1 and S2, andthe activation volume V_(act) illustrated in Table 2 were obtained.

Note that the coercive force Hc (25) and Hc (50) were set to the valuesillustrated in Table 2 by adjusting the thickness of the α-Fe layer inthe step of the reduction treatment of the particles A1 to A5 above, andalso by adjusting the air volume of the drying air while orienting themagnetic field during the film formation of the recording layer in therecording layer film formation step above. Also, the squareness ratiosS1 and S2 were set to the values illustrated in Table 2 by adjusting theair volume of the drying air while orienting the magnetic field duringthe film formation of the recording layer in the recording layer filmformation step above. Furthermore, the activation volume V_(act) was setto the values illustrated in Table 2 by adjusting the dispersion stateof the magnetic powder (particles A1 to A5) in the recordinglayer-forming coating preparation step above.

Example 6

In the recording layer film formation step above, after orienting themagnetic field of the magnetic powder in the lengthwise direction of thePEN film (the transport direction of the magnetic tape), the magneticfield of the magnetic powder was oriented in the thickness direction(perpendicular direction) of the PEN film. Additionally, the squarenessratios S1 and S2 were set to the values illustrated in Table 2 byadjusting the strength of the magnetic field during the magnetic fieldorientation and the air volume of the drying air in the recording layerfilm formation step. Besides the above, magnetic tapes were obtainedsimilarly to Example 2.

Example 7, and Comparative Example 3

In the recording layer film formation step above, after orienting themagnetic field of the magnetic powder in the width direction of the PENfilm, the magnetic field of the magnetic powder was oriented in thethickness direction (perpendicular direction) of the PEN film.Additionally, the squareness ratios S1 and S2 were set to the valuesillustrated in Table 2 by adjusting the strength of the magnetic fieldduring the magnetic field orientation and the air volume of the dryingair in the recording layer film formation step. Besides the above,magnetic tapes were obtained similarly to Example 2.

Examples 11 and 12, and Comparative Examples 4, 7, and 8

As illustrated in Table 2, besides using the powders of the particles B,C, D, E, and F instead of the powder of the particles A1, magnetic tapeswere obtained similarly to Example 1.

Comparative Example 5

As illustrated in Table 2, besides using the powder of the hexagonalplate-like barium ferrite (BaFe) particles instead of the powder of theparticles A1, a magnetic tape was obtained similarly to Example 1.

Comparative Example 6

As illustrated in Table 2, besides using the powder of needle-like Fe—Coalloy metal particles instead of the powder of the particles A1, amagnetic tape was obtained similarly to Example 1.

[Evaluation of Magnetic Tapes]

The magnetic tapes of Examples 1 to 12 and Comparative Examples 1 to 8obtained as described above were evaluated as follows.

(SNR in a 25° C. Environment)

A ½ inch tape transport device with an attached recording/reproductionhead and a recording/reproduction amp (MTS Transport, manufactured byMountain Engineering II, Inc.) was used to measure the SNR(electromagnetic conversion characteristics) of the magnetic tapes in a25° C. environment. For the recording head, a ring head with a gaplength of 0.2 μm was used, and for the reproduction head, a GMR headwith a shield-to-shield distance of 0.1 μm was used. The relative speedwas set to 6 m/s, and the recording clock frequency to 160 MHz. Also,the SNR was computed on the basis of the method described in thefollowing literature. As a result, the SNR of Example 4 is illustratedin Table 2 as a value relative to 0 dB.

Y. Okazaki: “An Error Rate Emulation System.”, IEEE Trans. Man., 31, pp.3093-3095(1995)

(Amount of SNR Degradation)

First, a ½ inch tape transport device with an attachedrecording/reproduction head and a recording/reproduction amp (MTSTransport, manufactured by Mountain Engineering II, Inc.) was used tomeasure the SNR in 25° C. and 50° C. environments. Note that themeasurement conditions and the method of computing the SNR were similarto the method of measuring the “SNR in a 25° C. environment” above.Next, the amount of SNR degradation was computed according to theformula below. The results are illustrated in Table 2.(Amount of SNR degradation) [dB]=(SNR measured at environmentaltemperature of 50° C.)−(SNR measured at environmental temperature of 25°C.)

Table 1 illustrates the configurations of the magnetic powders used inthe fabrication of the magnetic tapes of Examples 1 to 12 andComparative Examples 1 to 4, 7, and 8.

TABLE 1 Core part Average First shell part particle size (soft magneticlayer) Second shell of all Reduction part (oxide particles treatingThickness film) Particle type (nm) Material Material duration (H) (nm)Material Particle A1 20 ε iron oxide aFe 0.4 3 aFe₂O₃ Particle A2 0.32 2Particle A3 0.12 0.7 Particle A4 0.04 0.3 Particle A5 0.45 3.5 ParticleB 20 ε iron oxide aFe 0.32 2 Al₂O₃ Particle C 20 ε iron oxide Ni—Fealloy Sputtering 2 Al₂O₃ Particle D 20 ε iron oxide None — — NoneParticle E 17 Al-doped ε None — — None iron oxide Particle F 17 Ga-dopedε None — — None iron oxide

Table 2 illustrates the configurations and evaluation results of themagnetic tapes of Examples 1 to 12 and Comparative Examples 1 to 8.

TABLE 2 Hc (50)/Hc (25) × Amount of SNR SNR in 25° C. Magnetic Hc(50)Hc(25) 100 S1 S2 Vact degradation environment powder (kA/m) (kA/m) (%)(%) (%) S1/S2 (nm³) (dB) (dB) Example 1 Particle A1 200 210 95 25 25 14800 −1 0.2 Example 2 Particle A2 233 240 97 25 25 1 4800 −0.5 0.4Example 3 Particle A3 294 300 98 25 25 1 4800 0 0.8 Example 4 ParticleA2 228 240 95 30 30 1 4800 −0.5 0 Example 5 Particle A2 235 240 98 20 201 4800 0 0.6 Example 6 Particle A2 233 240 97 25 21 1.2 4800 0 0.6Example 7 Particle A2 230 240 96 25 28 0.9 4800 −0.5 0.4 Example 8Particle A2 228 240 95 25 25 1 2500 −0.5 0.9 Example 9 Particle A4 340350 97 25 25 1 4800 0 −1.2 Example 10 Particle A2 233 240 97 25 25 110000 0 −1.9 Example 11 Particle B 230 240 96 25 25 1 4800 −0.5 0.4Example 12 Particle C 230 240 96 25 25 1 4800 −0.5 0.4 ComparativeParticle A5 153 180 85 25 25 1 4800 −2 −0.5 Example 1 ComparativeParticle A2 192 240 80 35 35 1 4800 −2.5 −0.6 Example 2 ComparativeParticle A2 216 240 90 30 43 0.7 4800 −1.7 −1.7 Example 3 ComparativeParticle D 1238 1250 99 35 35 1 4800 — — Example 4 Comparative BaFe 175190 92 25 25 1 4500 −1.6 −0.3 Example 5 Comparative Metal 171 190 90 6060 1 5000 −1.8 −4.4 Example 6 Comparative Particle E 218 240 91 25 25 14800 −1.9 0.1 Example 7 Comparative Particle F 216 240 90 25 25 1 4800−1.7 0.1 Example 8

Hc (25): coercive force measured in the thickness direction(perpendicular direction) of the magnetic tape at an environmenttemperature of 25° C.

Hc (50): coercive force measured in the thickness direction(perpendicular direction) of the magnetic tape at an environmenttemperature of 50° C.

S1: squareness ratio measured in the lengthwise direction of themagnetic tape at an environmental temperature of 25° C.

S2: squareness ratio measured in the width direction of the magnetictape at an environmental temperature of 25° C.

V_(act): activation volume of the magnetic powder

From the above evaluations, the following was learned.

In Examples 1 to 12, because (a) the recording layer contains a powderof ε iron oxide particles, (b) the ratio R (=Hc(50)/Hc(25)×100) of thecoercive force Hc(50) and the coercive force Hc(25) is 95% or greater,(c) the coercive force Hc(25) is 200 kA/m or greater, and (d) thesquareness ratio S1 measured in the transport direction of the magneticrecording medium is 30% or less, the amount of SNR degradation withrespect to temperature change can be suppressed.

In Comparative Examples 1 to 8, 11, and 12, because (a) the recordinglayer contains a powder of ε iron oxide particles, (b) the ratio R ofthe coercive force Hc(50) and the coercive force Hc(25) is 95% orgreater, (c) the coercive force Hc(25) is 200 kA/m or greater and 340kA/m or less, (d) the squareness ratio S1 measured in the transportdirection of the magnetic recording medium is 30% or less, and (e) theactivation volume V_(act) is 5000 nm³ or less, the amount of SNRdegradation with respect to temperature change can be suppressed, whilein addition, a favorable SNR can be obtained at an environmentaltemperature of 25° C.

In Comparative Example 1, because the coercive force Hc(25) is less than200 kA/m, the temperature dependence of the coercive force Hc is large,the ratio R of the coercive force Hc(50) and the coercive force Hc(25)is less than 95%, and the amount of SNR degradation with respect totemperature change increases.

In Comparative Example 2, because the squareness ratios S1 and S2 in thelengthwise and width directions both exceed 30%, the perpendicularorientation is low, the ratio R of the coercive force Hc(50) and thecoercive force Hc(25) is less than 95%, and the amount of SNRdegradation with respect to temperature change increases.

In Comparative Example 3, because the squareness ratio S2 in the widthdirection exceeds 30%, the perpendicular orientation is low, the ratio Rof the coercive force Hc(50) and the coercive force Hc(25) is less than95%, and the amount of SNR degradation with respect to temperaturechange increases. Also, the SNR at an environmental temperature of 25°C. becomes worse.

In Comparative Example 4, because a shell layer is not formed, thecoercive force Hc(25) is too high, and signal recording is difficult.

In Comparative Examples 5 and 6, because a powder of barium ferrite(BaFe) particles and a powder of Fe—Co alloy metal particles are used asthe magnetic powder, the coercive force Hc(25) is less than 200 kA/m.Consequently, the temperature dependence of the coercive force Hc islarge, the ratio R of the coercive force Hc(50) and the coercive forceHc(25) becomes less than 95%, and the amount of SNR degradation withrespect to temperature change increases. Also, in Comparative Example 6,because the particles are needle-like, the squareness ratios S1 and S2in the lengthwise and width directions both greatly exceed 30%.Consequently, the perpendicular orientation is extremely low, and theSNR at an environmental temperature of 25° C. becomes much worse.

In Comparative Examples 7 and 8, the coercive force Hc is adjusted byadding A1 and Ga, but the temperature dependence of the coercive forceHc is large, the ratio R of the coercive force Hc(50) and the coerciveforce Hc(25) is less than 95%, and the amount of SNR degradation withrespect to temperature change increases.

The above specifically describes an embodiment and Examples of thepresent technology, but the present technology is not limited to theembodiment and Examples described above, and various modifications basedon the technical ideas of the present technology are possible.

For example, the configurations, methods, steps, shapes, materials,numerical quantities, and the like given in the embodiment and Examplesdescribed above are merely examples, and configurations, methods, steps,shapes, materials, numerical quantities, and the like that are differentfrom the above may also be used as necessary. Also, the chemicalformulas of compounds and the like are representative, and if a commonname for the same compound exists, the chemical formula is not limitedto the described valencies and the like.

In addition, it is also possible to combine the configurations, methods,steps, shapes, materials, numerical quantities, and the like of theembodiment and Examples described above with each other insofar as thecombination does not depart from the gist of the present technology.

Additionally, the present technology may also be configured as below.

(1)

A magnetic recording medium including:

a recording layer containing a powder of particles containing ε ironoxide, in which

a ratio ((Hc(50)/Hc(25))×100) of a coercive force Hc(50) measured in athickness direction of the magnetic recording medium at 50° C. and acoercive force Hc(25) measured in the thickness direction of themagnetic recording medium at 25° C. is 95% or greater,

the coercive force Hc(25) is 200 kA/m or greater, and

a squareness ratio measured in a transport direction of the magneticrecording medium is 30% or less.

(2)

The magnetic recording medium according to (1), in which the coerciveforce Hc(25) is 200 kA/m or greater and 340 kA/m or less.

(3)

The magnetic recording medium according to (1) or (2), in which theparticles have a core-shell structure.

(4)

The magnetic recording medium according to any one of (1) to (3),

in which the particles are provided with

a core part containing ε iron oxide, and

a shell part containing a soft magnetic material.

(5)

The magnetic recording medium according to (4),

in which the shell part is provided with

a first shell part provided on the core part, and

a second shell part provided on the first shell part,

the first shell part contains α-Fe, a NiFe alloy, or a FeSiAl alloy, and

the second shell part contains a iron oxide, aluminum oxide, or siliconoxide.

(6)

The magnetic recording medium according to any one of (1) to (5), inwhich an activation volume is 5000 nm³ or less.

(7)

The magnetic recording medium according to any one of (1) to (6), inwhich a squareness ratio measured in a width direction of the magneticrecording medium is 30% or less.

(8)

The magnetic recording medium according to any one of (1) to (7), inwhich the magnetic recording medium is used in a library device.

(9)

The magnetic recording medium according to any one of (1) to (8), inwhich the magnetic recording medium is used in a recording andreproduction device having a shortest recording wavelength of 75 nm orless.

(10)

The magnetic recording medium according to (9), in which the recordingand reproduction device is provided with a ring head as a head forrecording.

REFERENCE SIGNS LIST

-   11 Substrate-   12 Foundation layer-   13 Recording layer-   14 Backcoat layer-   15 Reinforcement layer-   16 Cupping suppression layer-   17 First reinforcement layer-   18 Second reinforcement layer-   19 Adhesion suppression layer-   21 Core part-   22, 23 Shell part-   22 a First shell part-   22 b Second shell part

The invention claimed is:
 1. A magnetic recording medium comprising: arecording layer containing a powder of particles containing ε ironoxide, wherein a ratio ((Hc(50)/Hc(25))×100) of a coercive force Hc(50)measured in a thickness direction of the magnetic recording medium at50° C. and a coercive force Hc(25) measured in the thickness directionof the magnetic recording medium at 25° C. is 95% or greater, asquareness ratio measured in a transport direction of the magneticrecording medium is 30% or less, wherein the coercive force Hc(25)ranges from 200 kA/m to 300 kA/m, and wherein an activation volume is5000 nm³ or less.
 2. The magnetic recording medium according to claim 1,wherein the particles have a core-shell structure.
 3. The magneticrecording medium according to claim 1, wherein the particles areprovided with a core part containing c iron oxide, and a shell partcontaining a soft magnetic material.
 4. The magnetic recording mediumaccording to claim 3, wherein the shell part is provided with a firstshell part provided on the core part, and a second shell part providedon the first shell part, the first shell part contains α-Fe, a NiFealloy, or a FeSiAl alloy, and the second shell part contains a ironoxide, aluminum oxide, or silicon oxide.
 5. The magnetic recordingmedium according to claim 1, wherein a squareness ratio measured in awidth direction of the magnetic recording medium is 30% or less.
 6. Themagnetic recording medium according to claim 1, wherein the magneticrecording medium is used in a library device.
 7. The magnetic recordingmedium according to claim 1, wherein the magnetic recording medium isused in a recording and reproduction device having a shortest recordingwavelength of 75 nm or less.
 8. The magnetic recording medium accordingto claim 7, wherein the recording and reproduction device is providedwith a ring head as a head for recording.
 9. The magnetic recordingmedium according to claim 1, wherein the coercive force Hc(25) rangesfrom 240 kA/m to 300 kA/m.
 10. The magnetic recording medium accordingto claim 1, wherein the activation volume ranges from 2500 nm³ to 4800nm³.